1. Field of the Invention
The present invention relates to an electronic device manufacturing apparatus. More particularly, the present invention relates to an electronic device manufacturing apparatus and a method for manufacturing an electronic device using such an apparatus, which are suitable for plasma excitation chemical vapor deposition (hereinafter, referred to as a "plasma CVD apparatus") or for plasma etching. Plasma CVD apparatuses are used in electronic industries for producing a semiconductor film, such as an amorphous silicon hydride thin film (hereinafter, referred to as an "a-Si:H thin film"), or an insulating film. Plasma etching apparatuses are used for processing a semiconductor device, a liquid crystal device, and the like.
2. Description of the Related Art
Plasma CVD apparatuses are used to deposit a thin film through plasma excitation and plasma dissociation of a material gas. On the other hand, plasma dry etching apparatuses are used to process a semiconductor device, a liquid crystal device, and the like. The plasma dry etching apparatuses operate based on a principle that plasma particles and active species generated by plasma excitation can be used to etch a film. Recently, these apparatuses have been widely used as electronic device manufacturing apparatuses for depositing/processing a metal film, a semiconductor film, a dielectric film, a crystalline silicon wafer, and the like.
Presently, many of these electronic device manufacturing apparatuses currently utilize a radio wave (about 13.56 MHz, also called an "RF" or an "HF"), or a microwave (about 2.45 GHz, also called an "MW") as an excitation frequency of a power source for generating the plasma.
On the other hand, regarding an excitation frequency of a high frequency power source for generating plasma, recent energetic studies in plasma science have been gradually elucidating the fact that frequencies in the range between the above-noted two frequencies (e.g., about 100 Mhz, also called a "VHF"), both theoretically and experimentally, have suitable characteristics for manufacturing electronic devices. Such studies are described in, for example, the following documents.
(1) J. Vac. Sci. Technol. A10 (1992) 1080, by A. A. Howling et al.
(2) Plasma Sources Sci. Technol. 2 (1993) p. 40-45, by T. Kitamura et al.
(3) Plasma Sources Sci. Technol. 2 (1993) p. 26-29, by S. Oda
(4) Japanese Laid-Open Patent Application No. 6-77144
One of the above suitable characteristics for manufacturing electronic devices is that plasma density increases in proportion to the square of the frequency used. That is, the film deposition rate (or the etching rate in the case of an etching apparatus) increases in proportion to the square to the frequency used. Another one of the above suitable characteristics is that the high plasma density is achieved at a relatively low plasma potential. This allows "plasma damages" (i.e., damages to a film or a substrate caused by ion species in plasma) to be suppressed even under such a high-speed deposition/etching condition.
In the field of electronic industries such as those called "giant microelectronics" (which involves the manufacturing of solar batteries and liquid crystal display devices using an a-Si:H thin film, or photosensitive layers of a photosensitive drum, etc.), the size of a substrate to be processed is large (for example, 40 cm to 60 cm in length). Accordingly, in order to achieve a higher throughput for an apparatus, it is becoming indispensable to provide a reaction chamber in which a plurality of such large substrates can be processed. Similarly, for semiconductor manufacturing apparatuses, it is very important to be able to process a number of substrates at a time in order to realize a high throughput. For these reasons, it is important to increase the size of reaction chamber and thereby to increase the apparatus size (i.e., the size of the reaction region, more particularly, the area of a cathode electrode and an anode electrode).
However, in the apparatuses described in the above documents (1) to (4), the reaction region is considerably small with respect to the wavelength of the high frequency voltage for plasma excitation. For example, for a frequency of 100 MHz, the wavelength is about 3 m, while the reaction region is only about 10 cm or less. Thus, so far, the reaction region of these apparatuses have not been made sufficiently large, and electronic device manufacturing apparatuses suitable for the field of electronic industries of giant microelectronics have not been realized.
According to the inventors of the present invention, the reason why the reaction region must be made so small with respect to the wavelength corresponding to an excitation power source frequency in the VHF range is as follows: when the scale of the reaction region becomes as great as the wavelength corresponding to the excitation power source frequency in the VHF range, electromagnetic waves generated therein begin to have characteristics as a wave which propagates across the reaction region. This causes a change in electromagnetic characteristics in the reaction apparatus. Such a change results in generation of structurally complicated plasma which cannot be controlled.
Hereinafter, such a phenomenon will be further described based on results of experiments conducted using a conventional plasma CVD apparatus 800 shown in FIG. 17. The electrode size of the plasma CVD apparatus 800 used was about 700 mm.times.700 mm.
When plasma was generated with the plasma CVD apparatus 800 using a frequency of about 13.56 MHz to about 20 MHz, electric discharge occurred only in a normal inter-electrode region, i.e., in a reaction region 83 defined between a cathode electrode 81 and an anode electrode 82 (see FIG. 18). However, it was confirmed that, when a frequency of about 27.12 MHz to about 35 MHz was used, electric discharge occurred not only in the reaction region 83, but also in locationally-abnormal regions (i.e., regions other than the normal inter-electrode region, which are irrelevant to film deposition). The locationally-abnormal regions include side regions 101 beside the reaction region 83 (i.e., regions defined between the side wall of the reaction chamber and either one end of the electrodes 81 and 82), and a region 102 behind the anode electrode 82.
It was also confirmed that, when a frequency was increased to about 40.68 MHz, electric discharge no longer occurred in the normal inter-electrode reaction region 83 and, instead, occurred only in the locationally-abnormal regions such as the side regions 101 and the region 102. As a result, an abnormal circumstance occurred, where no film was deposited on a substrate (or a wafer) on which a film is normally deposited, even though the substrate was properly placed on the surface of the anode electrode 82 in contact with the reaction region 83.
The inventors of the present invention made further examinations for such abnormal discharge while varying the size of the conventional plasma CVD apparatus 800. Among the apparatuses examined each having a square-shaped cathode electrode 81, a relatively small apparatus having an about 200 mm-by-200 mm cathode electrode 81 showed to cause discharge exclusively between electrodes with a frequency up to about 81.36 MHz. However, as can be seen from FIG. 19, as the size of the cathode electrode 81 increased, the upper limit of the frequency with which normal inter-electrode discharge was achieved, decreased.
When the size of the cathode electrode 81 was about 1200 mm by 1200 mm, normal inter-electrode discharge was achieved only with a frequency up to about 13.56 MHz. Moreover, with a frequency of about 40.68 MHz or higher, there only occurred locationally-abnormal discharge (i.e., discharge occurring in regions outside the reaction region between electrodes), so that a film was not deposited on the substrate placed on the surface of the anode electrode 82.
It was thus confirmed that, due to the locationally-abnormal discharge, it is not possible, using the conventional plasma CVD apparatus 800 with a frequency in the VHF range, to cause electric discharge over a large area for large-area film deposition.
As such locationally-abnormal discharge was analyzed using, as parameters, the value of the high frequency used for excitation and the size of the plasma CVD apparatus 800, it has been found that locationally-abnormal discharge occurs along with normal inter-electrode discharge in the range defined by Expression (4) below and as shown in FIG. 19. EQU D.gtoreq.(1/16).multidot..lambda. (4)
where D denotes a length of a side of a square-shaped cathode electrode
.lambda. denotes a wavelength (i.e., the velocity of light/frequency) of the high frequency voltage for excitation PA1 .mu. denotes a dielectric constant [Wb/A] PA1 .pi. denotes pi (ratio of the circumference of a circle to the diameter), and PA1 f denotes the frequency of the high frequency voltage used for excitation. PA1 .lambda. denotes a wavelength of the high frequency voltage for excitation. PA1 .lambda. denotes a wavelength of the high frequency voltage for excitation. PA1 .lambda. denotes a wavelength of the high frequency voltage for excitation. PA1 .pi. denotes pi (ratio of the circumference of a circle to the diameter), and PA1 f denotes the frequency of the high frequency voltage used for excitation; PA1 .lambda. denotes a wavelength of the high frequency voltage;
It was also confirmed that no discharge occurred between electrodes but, instead, discharge only occurred outside the inter-electrode region in the range of the electrode size D defined by Expression (5) below. EQU D.gtoreq.(1/8).multidot..lambda. (5)
Known methods for controlling plasma generated with a high frequency in the RF range are, for example, inserting a DC blocking capacitance element, or providing an impedance adjusting element around an electrode.
The former method is described in, for example, "Glow Discharge Processes" by John Wiley & Sons (1980) B. Chapmann, while the latter is described in, for example, Japanese Laid-Open Patent Application No. 58-145100 and Japanese Laid-Open Patent Application No. 6-61185.
The DC blocking capacitance element as an impedance adjusting element is normally inserted into a location outside the reaction apparatus, where the element is remote from the cathode electrode. In particular, according to the method described in Japanese Laid-Open Patent Application No. 58-145100, the impedance adjusting element is inserted between internal sections of a cathode electrode. On the other hand, according to the method described in Japanese Laid-Open Patent Application No. 6-61185, the element is inserted between the ground potential and an electrode opposing the cathode electrode.
However, these techniques can only be applied to methods which aim to control plasma generation on the assumption that a high frequency in the RF range is used. As these methods are applied to an electronic device manufacturing apparatus which uses a high frequency in the VHF range, the above-mentioned problems cannot be solved for the following reasons.
For example, consider a capacitance-coupled electronic device manufacturing apparatus whose cathode and anode electrodes are parallel-plate electrodes and which is a large-scale apparatus having a reaction region or an electrode having a dimension of about 1 m. As shown in FIG. 17, in order to deposit or etch a film using a capacitance-coupled apparatus, plasma is generated in a region 83 (containing a material gas) between the cathode electrode (high frequency excitation electrode) 81 and the anode electrode 82 opposing the cathode electrode 81 and having a ground potential for a DC voltage, so as to dissociate the material gas.
When the high frequency for excitation is in the RF range, the impedance between the electrodes 81 and 82 can be considered as a capacitance component. In such a case, plasma is generated between the electrodes 81 and 82, so that film deposition is normally achieved.
However, when a frequency in the VHF range is used, the high frequency for excitation begins to have characteristics as an electromagnetic wave which propagates across the reaction region. Accordingly, a group of conductors surrounding the reaction region begins to have an inductance component, so that, at a certain frequency, parallel resonance occurs between the conductor and a floating capacitance of the cathode electrode 81. Then, the impedance in the region between the electrodes 81 and 82 becomes considerably large so that the inter-electrode region equivalently becomes an infinite space. In such a case, it is difficult to generate plasma between the electrodes 81 and 82.
In order to solve this problem, it is necessary to control the impedance between a portion of the apparatus which has the ground potential and the cathode electrode 81. It is difficult to do so based on methods such as the conventional control method of inserting a DC blocking capacitance element, or the method described in Japanese Laid-Open Patent Application No. 58-145100 or Japanese Laid-Open Patent Application No. 6-61185.
As shown in FIG. 17, the DC blocking capacitance element 87 formed from a capacitor is inserted in series between the cathode electrode 81 and a high frequency power source (high frequency power generator) 84. Therefore, it is not possible to control the value of the floating capacitance discussed above. Moreover, in accordance with the method described in Japanese Laid-Open Patent Application No. 58-145100, the impedance between the cathode electrode and an external circuit does not vary essentially. Furthermore, in accordance with the method described in Japanese Laid-Open Patent Application No. 6-61185, the impedance of the cathode electrode side cannot be controlled since the impedance adjusting element is provided on the side of the anode electrode.
More particularly, in the plasma CVD apparatus 800 shown in FIG. 17, considering the propagation of a high frequency in the RF range, the floating capacitance component C.sub.F [C/V=F] occurs mainly below the cathode electrode 81, and the reaction region (or inter-electrode region) 83 between the cathode electrode 81 and the anode electrode 82 becomes an inductance equivalent L.sub.G [Wb/A=H].
Herein, the floating capacitance C.sub.F is given by Expression (8) below. EQU C.sub.F =.epsilon..multidot.S.sub.1 /d.sub.0 (8)
where d.sub.0 denotes a distance between surfaces of the cathode electrode and the anode electrode opposing each other
Using short-circuited waveguide approximation, the inductance L.sub.G is given by Expression (9) below. EQU L.sub.G =A.times.tan {(2.pi..multidot.f.multidot.S.sub.2 /c)}/(2.pi..multidot.f) (9)
where .epsilon. denotes a dielectric constant [C/V.multidot.m]
S.sub.1 denotes an opposing area [m.sup.2 ] of the cathode electrode and the anode electrode
d denotes a distance [m] between the cathode electrode and the anode electrode
f denotes a frequency [1/S]
S.sub.2 denotes the length [m] of the inter-electrode region along the electrode plane, and
c denotes the velocity of light [m/S]
"A" in the above Expression (9) is a constant which can be expressed by Expression (10) below. EQU A=(d/W).multidot..sqroot.(.mu./.epsilon.) (10)
where W denotes a width of electrode [m]
Since the floating capacitance C.sub.F and the inductance L.sub.G are in parallel connection between the cathode electrode 81 and the ground level as can be seen from FIG. 17, when the frequency f is equal to the parallel resonance frequency f.sub.0 defined by Expression (11) below, parallel resonance occurs, and the impedance in the reaction region 83 between the electrodes 81 and 82 increases to infinity. EQU f.sub.0 =1/{2.pi..multidot..sqroot.(L.sub.G C.sub.F)} (11)
That is, when the frequency f of the excitation high frequency is equal to or approximately equal to the parallel resonance frequency f.sub.0, plasma generation between the electrodes 81 and 82 cannot be expected.
Thus, there occurs a need to control the impedance in the reaction region 83. However, since the size of the reaction region 83 is determined by the size of the substrate on which a film is deposited, it is practically difficult to vary the magnitude of the inductance L.sub.G of the reaction region 83.
For the reasons above, there has been a restriction in increasing the apparatus size (i.e., the size of the reaction region) of the conventional electronic device manufacturing apparatus which uses a high frequency in the VHF range. Accordingly, it has not been possible to improve the mass-productivity of electronic devices.